1. Field of the Invention
The present invention relates generally to wafer processing methods and, more particularly, to a method for planarizing unevenness on a surface of a wafer photoresist layer and also to a wafer produced by the method
2. Description of the Related Art
Micro-fluid ejection heads are useful for ejecting a variety of fluids including inks, cooling fluids, pharmaceuticals, lubricants and the like. One use of micro-fluid ejection heads is in an ink jet printer. Ink jet printers continue to be improved as the technology for making the micro-fluid ejection heads continues to advance. New techniques are constantly being developed to provide low cost, highly reliable printers which approach the speed and quality of laser printers. An added benefit of ink jet printers is that color images can be produced at a fraction of the cost of laser printers with as good or better quality than laser printers. All of the foregoing benefits exhibited by ink jet printers have also increased the competitiveness of suppliers to provide comparable printers in a more cost efficient manner than their competitors.
One area of improvement in the printers is in the micro-fluid ejection head itself. This seemingly simple device is a relatively complicated structure containing electrical circuits, ink passageways and a variety of tiny parts assembled with precision to provide a powerful, yet versatile micro-fluid ejection head. The components of the ejection head must cooperate with each other and with a variety of ink formulations to provide the desired print properties. Accordingly, it is important to match the ejection head components to the ink and the duty cycle demanded by the printer. Slight variations in production quality can have a tremendous influence on the product yield and resulting printer performance.
In order to improve the quality of the micro-fluid ejection heads, new techniques for fabricating components of the heads are being developed. For example, electrostatic chucks (ESC) are being used to hold a wafer during Deep Reactive Ion Etching (DRIE) and other wafer processing steps. See the U.S. patent application cross-referenced above as well as U.S. patent application publication no. 2007/0004215 to Mrvos et al. Both are assigned to the assignee of the present invention and their disclosures are hereby incorporated herein by reference. See also U.S. Pat. No. 6,628,500 to Thomas et al. DRIE is used in various ways including to form ink vias in the wafer to provide fluid to ejection actuator devices on a device surface of the wafer. However, DRIE generates heat that can adversely affect components of the micro-fluid ejection head, particularly organic photoresist layers on the ejection head substrate used as masking layers and/or etch stop layers.
The ESC generates a uniform electrostatic capacitively inducted force that clamps a wafer to a dielectric (ceramic plate) on an electrode of the ESC for supporting the wafer during the etching process. The etch stop constituted by a positive photoresist coating or layer formed on a back side of the wafer facing the ESC dielectric substrate prevents the etching from reaching the dielectric through the front side of the wafer. Due to the process used in forming it, the etch stop is typically an irregular or non-uniform film layer with a non-uniform edge bead provided about the perimeter of the layer.
Wafers need to be cooled during the etching process, either by convection or conduction techniques or a combination of both. A significant portion of the wafer clamping force and wafer cooling quality is due to a gap distance between the wafer and the dielectric. In the past, an ESC was used having a series of mesas on the front side of its dielectric to define a three dimensional space between it and the etch stop layer in order to create the necessary helium circuit for wafer cooling and to control the amount of area of direct electrical contact between the wafer and the ESC. See, for an example, U.S. Pat. No. 5,583,736 to Anderson et al. As long as the ESC used had mesas, the presence of the edge bead of the etch stop was found to be beneficial by contributing to enhanced cooling of the wafer through increasing the amount of surface area contacted by the helium as well as increased volume of helium used to remove heat at the edge of wafers where damage traditionally is incurred from wafer cooling varying negatively. However, significant non-uniformity of the edge bead is problematic for clamping uniformity and helium containment.
Recent improvements in ESC construction, namely the use of a thick higher purity dielectric ESC (hereinafter called an improved ESC), has improved clamping force and wafer cooling by improving, among other things, the thermal conductivity of the dielectric, the voltage across the dielectric, reducing the gap distance between the wafer and the dielectric, and changing the helium cooling circuit. Furthermore, the mesas have been eliminated from the dielectric and a single helium back side gas circuit has been provided at the edge of the underside of the wafer.
Where the irregular edge bead on the etch stop was beneficial to wafer cooling as long as mesas were provided on the ESC dielectric since the edge bead offset the negative effect on the clamping force quality of the mesas, with the mesas now eliminated the presence of the irregular edge bead now presents significant challenges to helium back side gas containment and wafer cooling in a system using the improved ESC dielectric. The recent improvement in ESC construction thus means that the presence of edge bead on the etch stop is no longer beneficial. The edge bead presence is now a problem for the DRIE etch process because it increases gap distance between the wafer and the improved ESC, negatively affecting both cooling and clamping of the wafer.
Thus, edge bead removal from the etch stop is now desired. However, employment of known mechanical and chemical removal methods on the edge bead exacerbate the problem. Both methods, though highly optimized using available equipment and knowledge, are fundamentally limited by the presence of a main flat feature on the wafer. This limitation causes positive resist features that make the efficient cooling of the wafer during the etch step more difficult. Current edge bead removal techniques present challenges from a process control standpoint because of this variation across the wafer between the main flat area and the rest thereof.
Thus, there is a need for an innovation that will be effective for planarizing a sacrificial photoresist layer, such as the etch stop with an edge bead, in order to maximize wafer clamping and cooling and thereby improve the quality of the polymer disposition and etch in a DRIE etch process.